The present invention relates to the field of optical fiber cables, and, more particularly, to optical fiber cables containing optical fibers with chromatic dispersion characteristics designed to be incorporated in a dispersion managed telecommunications system intended for high data rate, long distance signal transmission.
Optical fiber cables are used in telecommunication systems to transmit telephone, television, and computer data information in indoor and outdoor environments. The distance and data rate capabilities of these systems are directly affected by the chromatic dispersion performance of the optical fibers contained within the cables making up the system. From a technical perspective, the chromatic dispersion performance of each optical fiber is defined by the sum of the material dispersion and waveguide dispersion inherently present due to the material makeup and engineering design of the optical fiber. Material dispersion refers to the dispersion imposed upon a transmitted signal caused by the material composition of the medium through which it is traveling (silica glass is most commonly utilized in optical telecommunications). Changes in refractive index with wavelength give rise to a range of material dispersion values in a single optical fiber. In glass (silica) optical fibers, material dispersion increases with wavelength over a wavelength range of about 0.9 xcexcm to 1.6 xcexcm. The material dispersion attribute for an individual optical fiber can be characterized as xe2x80x9cnegativexe2x80x9d or xe2x80x9cpositivexe2x80x9d depending on the wavelength of the transmitted signal. Waveguide dispersion refers to the dispersion imposed upon a transmitted signal caused by the design and construction of the medium through which it is traveling. Doping is most commonly done by doping portions of the optical fiber with a specific material, commonly Germania, to create a specific index of refraction profile. The specific value of waveguide dispersion is also wavelength dependent and gives rise to a range of wavelength dispersion values in a single optical fiber. Waveguide and material dispersion effects are typically summed yielding an overall positive or negative chromatic dispersion characteristic in a given optical fiber.
An optical fiber cable design that incorporates chromatic dispersion affects is described in U.S. Pat. No. 5,611,016. The patent pertains to a dispersion-balanced optical cable for reducing four-photon mixing in Wave Division Multiplexing systems, the cable being designed to reduce cumulative dispersion to near zero. The dispersion-balanced optical cable requires positive and negative dispersion fibers in the same cable. Further, the positive dispersion aspect includes a dispersion characteristic defined as the average of the absolute magnitudes of the dispersions of the positive dispersion fibers exceeding a magnitude of 0.8 ps/nmxc2x7km at a source wavelength. In addition, the negative dispersion optical fiber characteristic requires the average of the absolute magnitudes of the dispersions of the negative dispersion to exceed a magnitude of 0.8 ps/nmxc2x7km at the source wavelength.
The aforementioned optical fibers are single-mode optical fibers designed for the transmission of optical signals in the 1550 nm wavelength region. At defined parameters, the positive-dispersion characteristic is +2.3 ps/nmxc2x7km and the negative-dispersion characteristic is xe2x88x921.6 ps/nmxc2x7km. The aforementioned patent describes an optical ribbon including both positive and negative dispersion compensated optical fibers, enclosed within a mono-tube cable. A light-colored portion of the optical ribbon contains positive-dispersion optical fibers whereas a dark-colored portion of the optical ribbon contains negative-dispersion optical fibers. Two kinds of optical fibers are required in the same mono-tube.
Aspects of the Invention
An optical fiber cable for use in a dispersion managed cable system (DMCS) contains optical fibers that exhibit a carefully controlled chromatic dispersion performance to support long distance, high data rate transmission, and buffer tubes in the cable segregating groups of these optical fibers, at least some of the buffer tubes having at least one DMCS identification marking thereon and respectively containing at least one of the optical fibers having a carefully controlled chromatic dispersion performance to support long distance, high data rate transmission, the cable further including a protective outer cable jacket with at least one DMCS identification marking thereon.
In another aspect of the present invention, an optical fiber cable for use in a DMCS having optical fibers, at least some of the optical fibers having a carefully controlled chromatic dispersion performance to support long distance, high data rate transmission and buffer tubes segregating groups of optical fibers, at least some of the buffer tubes having at least one DMCS identification marking thereon and respectively containing at least one of the optical fibers having a carefully controlled chromatic dispersion performance to support long distance, high data rate transmission, the DMCS identification system of the present invention is not necessarily based on the convention of smallest to largest optical signal transmission zone. This zone is defined by the Mode Field Diameter (MFD) in single-mode optical fibers and the core diameter in multimode optical fibers. Rather, the DMCS identification system of the present invention has fibers arranged in the following order: DMCS optical fibers are first in the sequence order, then the remaining optical fiber types in order of increasing MFD/core diameter in subsequent respective buffer tubes. The buffer tubes are arranged such that DMCS identification marked buffer tubes with either positive or negative dispersion optical fibers are first in the sequence of buffer tubes contained in the cable, and buffer tubes containing non-DMCS optical fibers (if included) are arranged to follow the DMCS buffer tubes. If more than one type of non-DMCS optical fiber type is included the sequence order will be based upon increasing Mode Field Diameter/Core Diameter of the optical fibers contained in each buffer tube. In cables that require multilayers of buffer tubes, the DMCS buffer tubes will preferentially be located in the innermost layer of buffer tubes.
In another aspect of the present invention, an optical fiber cable for use in a DMCS is defined having one or more layer(s) of buffer tubes and those buffer tubes arranged in a predetermined sequence for identification purposes. The first set of buffer tubes containing a first set of optical fibers with predetermined positive or negative chromatic dispersion characteristic respectively therein having a dispersion such that the range of absolute values of the chromatic dispersion is between about ten to about forty ps/nm.km; and a second set of buffer tubes containing other types of optical fibers having varying MFD/Core Diameters; the first and second sets of buffer tubes being separated by at least one interposed cable component.